The invention disclosed and claimed herein generally pertains to an improved method for reducing artifacts in a magnetic resonance (MR) image acquired by means of a phased array surface coil. More particularly, the invention pertains to a method of the above type wherein only a subset, i.e., fewer than all, of the coil elements forming the array are used to acquire the MR data needed for the image. Even more particularly, the invention pertains to a method of the above type wherein an MR scanner or system employed to implement the method of the invention can be readily programmed by an operator to automatically select the respective coil elements of the subset which are to be used in acquiring the MR data.
As is well known to practitioners in the field of MR imaging, phased array surface coils have been developed to overcome certain deficiencies in other types of MR receivers. More specifically, a phased array coil provides an MR receiver which generally has better signal to noise ratio than a volume coil receiver, and at the same time does not diminish field of view, as tends to happen with a surface coil. Phased array receivers have been found to be particularly useful in imaging elongated structures, such as the cervic, thoracic and lumbar regions of the spine.
As is further well known, phased array surface coils are subject to artifacts which can occur in MR imaging whenever necessary magnetic field values, such as the B.sub.0 or gradient fields, are replicated or repeated at more than one location within an MR signal source (i.e., within an imaging subject). Such artifacts derive primarily from the fact that gradient and magnetic coils have finite lengths. Accordingly, the B.sub.0 and gradient fields of an MR scanner become increasingly non-linear away from the isocenter of the scanner main magnet. For example, the ideal G.sub.z gradient field, which is linear with respect to the Z-axis, should have a different value at each Z-axis position. However, because of the non-linearity, the G.sub.z field could be the same at two widely separated Z-axis positions, on opposing sides of the isocenter. As a result, MR signals detected at both positions by a receiver could be accepted for use in constructing an image, even though one of the positions was actually outside the image field of view. The MR signal from such position would thereby cause an artifact.
In a phased array surface coil, such peripheral signal artifacts can appear as either a bright spot or as a ribbon of signal smeared through the image. The bright spot artifacts are frequently referred to as Startifacts, and the ribbon artifacts may be referred to as Annefacts.